Don't emit .gnu_pubnames when tuning for LLDB.

LLDB reads the various .apple* accelerator tables (and in the near
future: the DWARF 5 accelerator tables) which should make
.gnu_pubnames redundant. This changes the Clang driver to no longer
pass -ggnu-pubnames when tuning for LLDB.

Thanks to David Blaikie for pointing this out!
http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20190422/thread.html#646062

rdar://problem/50142073

Differential Revision: https://reviews.llvm.org/D67373

llvm-svn: 371530